The present invention relates to a method of producing a ball valve to be connected to a fluid piping system, and more particularly to a method of assembling a housing, adapted to contain a ball therein, by welding.
(1) U.S. Pat. No. 4,265,427 (corresponding to JP-A-55-14394) discloses one example ball valve which comprises, as main component members, a ball, and a housing containing the ball.
In the ball valve, a spherical shut-off member (i.e. a ball), contained in a hood (or a housing), has a pair of projections (or stems), and the projections are rotatably supported by a pair of support rings attached to an inner surface of the hood, respectively. The hood is assembled by welding a pair of hood halves together, and butt welding end faces of the pair of hood halves lie in a plane including an axis of rotation of the spherical shut-off member. Namely, the pair of hood halves are mirror-symmetrical with respect to the plane including the axis of rotation of the spherical shut-off member.
The hood has an operation hole (or window) provided for rotating the spherical shut-off member around the axis, and hemi-circular recesses, corresponding to the operation hole, are formed respectively in the pair of hood halves before the two hood halves are jointed together by welding.
The pair of support rings serve to receive a thrust force from a fluid, are provided respectively at opposed positions on the inner surface of the hood, and are disposed near to the axis of rotation of the spherical shut-off member in surrounding relation thereto, wherein one of the support rings surrounds the operation hole. Each of the two support rings comprises a combination of support ring halves corresponding respectively to the pair of hood halves, and the support ring halves are secured respectively on the inner surfaces of the pair of hood halves by welding prior to welding of the hood halves. Therefore, when the pair of hood halve are jointed together by welding, each pair of support ring halves are abutted against each other at their butting end faces to form the circular ring.
In order to prevent the inside of the hood from being affected by the welding operation (by which the pair of hood halves are jointed together), a partially cut off grooved ring is attached by welding on the inner surface adjacent to the welding face (i.e. the butt welding end face) of one of the hood halves. The grooved ring extends toward the mating hood half in the direction of the width thereof so as to extend circumferentially along the inner side of the butted end faces of the pair of hood halves for welding.
In the ball valve having such a structure, since the pair of support rings, which receive a thrust force from fluid, and the operation hole of the hood, are located close to the butt welded faces of the pair of hood halves, there is a possibility that thermal strain is induced in the hood due to welding heat resulting in a displacement of each support ring half from its proper position. In this case, the precision of the axis of rotation of the spherical shut-off member is affected, so that incomplete contact between valve seats, which are provided on the inner surface of the hood, and the spherical shut-off member occurs.
(2) U.S. Pat. No. 4,235,003 discloses a method of producing a ball valve. In the disclosed ball valve, a ball is contained in a hollow member (i.e. a housing), and the ball is rotatably supported by a pair of rings (i.e. valves seats). In the ball valve having such structure, unlike the ball valve disclosed in U.S. Pat. No. 4,265,427, the rings receive a thrust force from fluid.
The hollow member of the ball valve, disclosed in U.S. Pat. No. 4,235,003, is formed by jointing a pair of symmetrical pipe members together by welding. Each pipe member has a hemi-spherical enlarged portion (of a bowl-shape) formed at one end thereof, and open end faces of the two enlarged portions are butted and welded together at the end faces, thereby forming the hollow member. When the ball valve is opened or closed, the ball within the hollow member is operated by an operation stem passing through a hole formed in the hollow member, the hole being formed by a combination of hemi-circular recesses which are formed respectively in the open ends of the two enlarged portions prior to the welding for obtaining the hollow member. Here, the remaining portion of each pipe other than the enlarged portion will be referred to as a straight pipe portion, and a sleeve of a wedge-shaped cross-section for holding a ring (i.e. a valve seat) is fitted into each of the straight pipe portion, and is fixedly secured thereto by welding.
In the ball valve, although the straight pipe portions have an inner diameter corresponding to that in a fluid piping, there is a disadvantage that the sleeves and the rings (i.e. valve seats) narrow a fluid passage. Further, since the hemi-circular recesses are formed respectively in the enlarged portions of the two pipe members prior to welding for obtaining the hollow member, there is a possibility that the operating stem-passing hole, formed by the combination of the hemi-circular recesses, is affected by thermal strain which is induced in the hollow member by welding, so that the center of the hole is displaced out of a proper position.
(3) A third example of known ball valves is shown in FIG. 5. The ball valve comprises, as main components, a housing 01, having openings 02A and 03A formed respectively at opposite sides thereof, a ball 05, which is contained in the housing, and has a central through bore 05A, and a pair of stems 07 and 08 supporting the ball. The ball 05 is rotated by the operation stem 08 about the axis of the both stems. There are provided valve seat rings 06 and 06 between the housing 01 and the ball 05 which is kept always in contact with the valve seat rings 06 and 06.
The housing 01 consists of three members, that is, a first member 02, a second member 03 and a third member 04, the first and second members being mirror-symmetrical. The three members each having a generally tubular shape are butted and welded (W) at their open end faces.
The stems 07 and 08 extend through the third member 04. The stem 07 extends through the third member 04 and is fixed thereto. It is also rotatably fitted in a bore for stem of the ball 05. The other stem 08 is of an operation stem for rotating the ball valve, which rotatably extends through the third member 04, is supported by a separate support member 09, and is fitted in a bore for stem of the ball 05 so as not to rotate relatively to the ball.
This ball valve is of such a structure that the pair of stems 07 and 08 receive a thrust force from fluid. The housing 01 is assembled by jointing the three sectional members together by welding, which is disadvantageous from an economical point of view since the number of the components is large. Besides, since the two weld lines W, W are close to the valve seat rings 06 and 06, respectively, there will occur an unconformity with respect to the contact relationship between each of the valve seat rings 06, 06 and the ball 05 under the influence of thermal strain induced in the housing. Further, since the weld lines W, W are close to the stems 07 and 08, the shape of the respective holes of the third member 04 for the stems is affected by the thermal strain, whereby the precision of the stems 07 and 08, that is, the axis of rotation of the ball 05, is deteriorated, also resulting in an unconformity with respect to the contact relationship between each of the valve seat rings 06, 06 and the ball 05.
The present invention has been proposed under the above technical background.
Problems to be solved by the invention are to reduce the number of components, in producing a so-called trunnion type ball valve, in order to save the production cost for the trunnion type ball valve, such as the first conventional example of U.S. Pat. No. 4,265,427 and the third conventional example shown in the accompanying drawing of FIG. 5, and to eliminate adverse effects of thermal strain induced in a housing due to welding whereby obtaining the ball valve with a high precision.
According to the present invention, there is provided the following ball valve producing method:
a ball valve to be produced is connected to a fluid piping system, and comprises a valve housing (for example, a wall thickness=10 to 50 mm), and a ball rotatably contained in the valve housing. The valve housing and the ball are made, for example, of carbon steel or austenitic stainless steel (JIS SUS304).
The valve housing has a pair of holes, which are located in opposed positions so as to be connected respectively to pipes constituting the piping system, and a pair of stem assemblies which are located in opposed positions on a straight line perpendicularly intersecting another straight line, passing through centers of the pair of openings and a center of the ball, at the center of the ball.
The ball has a through bore, which can be brought into alignment with the pair of openings to allow the passage of a fluid therethrough, and a pair of recessed portions receiving the pair of stem assemblies, respectively. The ball is rotatable within the valve housing about a centerline passing through the pair of stem assemblies.
The ball valve having the above structure is produced by the following process:
(1) A pair of housing halves for forming the valve housing, and the ball having the through bore whose opposite open ends are formed to be flat, are prepared. The pair of housing halves are so formed to be mirror-symmetrical bodies of which forms are defined by sectioning the valve housing at the middle between the pair of openings. The ball is previously provided with a pair of recessed portions.
(2) Annular valve seats, each forming a seal between an outer surface of the ball and an inner surface of the valve housing, are mounted in the pair of housing halves, respectively.
(3) The pair of housing halves are butted at their open ends (opposite to the openings, respectively) so as to enclose the ball. At this time, the ball is kept in such a position that the ball is turned by an angle of 90 degrees from a normal operational position of the ball valve, while causing the center-line of the through bore to be in alignment with the operational axis of the ball and to perpendicularly intersect the straight line passing through the centers of the pair of openings and the center of the ball.
(4) The butted open ends of the pair of housing halves are jointed together by welding, thereby forming the valve housing.
(5) Without changing the position of the ball, holes for mounting the pair of stem assemblies respectively, are formed by machining at those positions of the valve housing faced respectively to the both flat open ends of the through bore of the ball. The both holes are so formed that the centers thereof are located on the operational centerline of the ball valve.
(6) The ball is turned by an angle of 90 degrees to be in the normal operational state of the ball valve such that the through bore is aligned with the both holes of the valve housing and that the recessed portions of the ball are aligned with the both holes of the housing, respectively. Next, the pair of stem assemblies are attached to the both holes, respectively, and stems are fitted into the both recessed portions, respectively, and subsequently the pair of stem assemblies are welded circumferentially to edge portions of the holes, respectively.
An embodiment of the invention method characterized by the above steps of the process will be described in the following:
(1) The welding at step (4) is effected by a TIG welding method (i.e. Tungsten inert-gas arc welding). In the TIG welding method, since no flux is used, spattering of slag will not occur during the welding operation, so that the contamination of the interior of the valve housing by foreign matters can be effectively prevented. Here, it should be noted that the entry of foreign matters, which would adversely affect the condition of contact of the ball with the valve seats, should be positively avoided.
(2) One of the pair of stem assemblies includes a non-operation stem, fitted in one of the two recessed portions, and an auxiliary member, and a base portion of the auxiliary member is welded circumferentially by welding to the peripheral edge of one of the holes. The other of the pair of stem assemblies includes a sleeve, fixedly secured to the valve housing, and a valve stem (i.e. valve-operating stem) rotatably fitted in the sleeve, with a seal ring interposed therebetween, and in a condition in which the valve stem is fitted in the sleeve, a distal end portion of the valve stem is fitted in the other recessed portion in a manner to prevent a relative rotation therebetween, and an end portion of the sleeve is jointed by welding to the peripheral edge of the hole over an entire periphery of the sleeve end portion.
(3) A flange plate is detachably fixed to a free end of the sleeve to prevent the valve stem from moving axially out of engagement with the sleeve, and a free end portion of the valve stem extends outwardly from the flange plate through an opening in the flange plate. This outwardly-extending portion is used as a valve-operation stem for rotating the ball about the centerline of the ball to open and close the ball valve.
(4) The pair of valve seats are mounted respectively on the inner surfaces of the housing halves in such a manner that the valve seats are located close to the pair of openings in parallel with end surfaces of the openings, respectively.
(5) Each of the pair of valve seats has a resilient member, and the valve seat is brought to close contact with the outer surface of the ball by a spring force of the resilient member.
(6) A body of the valve seat is a metal ring, and a resin ring is mounted on a peripheral edge of the metal ring, and the resin ring is brought to close contact with the outer surface of the ball.
(7) A body of the valve seat is a metal ring, and a rubber ring is mounted on a peripheral edge of the metal ring, and the rubber ring is brought to close contact with the outer surface of the ball.